1. Field of the Invention
The present invention relates to a structure of a display device having transistors. In particular, the invention relates to a structure of an active matrix display device having thin film transistors manufactured over an insulator such as glass or plastic. Further, the invention relates to an electronic device using such a display device as a display portion.
2. Description of the Related Art
In recent years, a so-called self-luminous display device having pixels each of which includes a light-emitting element such as a light-emitting diode (LED) has been drawing attention. As a light-emitting element used for such a self-luminous display device, an organic light-emitting diode (also called an OLED (Organic Light-Emitting Diode), an organic EL element, an electroluminescence (Electro Luminescence: EL) element, and the like) is drawing attention, which is becoming to be more frequently used for an EL display and the like. Since a light-emitting element such as an OLED is a self-luminous type, an EL display has advantages that visibility of pixels is higher, no backlight is required, the response speed is higher, and the like, compared with a liquid crystal display. The luminance of a light-emitting element is controlled with the amount of current flowing thereto.
In addition, in recent years, an active matrix display device having pixels each of which is provided with a light-emitting element and a transistor for controlling the light emission of the light-emitting element has been developed. An active matrix display device is expected to be put into practical use because it can realize high-resolution display on a large screen, which is difficult to be achieved with a passive matrix display device, and realize an operation with lower power consumption than that of a passive matrix display device, as well as having high reliability.
When driving methods of pixels of an active matrix display device are classified according to the kind of input signals, a voltage programming method and a current programming method can be given as examples. The former voltage programming method is a method of controlling the luminance of a light-emitting element with a driving element by inputting a video signal (voltage) which is to be delivered to a pixel to a gate electrode of the driving element. On the other hand, the latter current programming method is a method of controlling the luminance of a light-emitting element by flowing a programmed signal current into the light-emitting element.
An exemplary pixel configuration of a display device which employs a voltage programming method, and a driving method thereof will be described, with reference to FIG. 64. Note that description will be made by using an EL display device as a typical display device.
FIG. 64 is a diagram illustrating an exemplary pixel configuration of a display device which employs a voltage programming method (see Reference 1: Japanese Published Patent Application No. 2001-147659). A pixel shown in FIG. 64 includes a driving transistor 6401, a switching transistor 6402, a storage capacitor 6403, a signal line 6404, a scan line 6405, a first power supply line 6406, a second power supply line 6407, and a light-emitting element 6408.
Note that in this specification, the description that “a transistor is on” means a state that a gate-source voltage of a transistor is higher than the threshold voltage thereof, and thus a current flows between a source and a drain, while the description that “a transistor is off” means a state that a gate-source voltage of a transistor is less than or equal to the threshold voltage thereof, and thus no current flows between a source and a drain.
When the switching transistor 6402 is turned on in response to a potential change of the scan line 6405, a video signal which is input to the signal line 6404 is delivered to a gate of the driving transistor 6401. A gate-source voltage of the driving transistor 6401 is determined by a potential of the video signal input, and a current flowing between a source and a drain of the driving transistor 6401 is determined accordingly. This current is supplied to the light-emitting element 6408, and thus the light-emitting element 6408 emits light.
In this manner, the voltage programming method is a method of setting the gate-source voltage of the driving transistor 6401 with a potential of a video signal, and thus setting a current flowing between the source and the drain of the driving transistor 6401, so that the light-emitting element 6408 emits light at a luminance corresponding to the current.
As a semiconductor element for driving a light-emitting element, a polysilicon (p-Si) transistor is used. However, a polysilicon transistor can easily have variations in electrical properties such as the threshold voltage, on-current, and mobility, resulting from defects in crystal grain boundaries. Referring to the pixel shown in FIG. 64, when the properties of the driving transistor 6401 vary between each pixel, the drain current thereof also varies even when the same video signal is input. Accordingly, the luminance of the light-emitting element 6408 varies.
In addition, in the conventional pixel circuit (FIG. 64), a storage capacitor is connected between the gate and the source of the driving transistor. Given that this storage capacitor is formed from a MOS transistor, a channel region of the MOS transistor cannot be induced when a gate-source voltage of the MOS transistor becomes substantially equal to the threshold voltage thereof Therefore, the MOS transitor cannot function as a storage capacitor. As a result, video signals cannot be accurately held therein.
In this manner, in the conventional voltage programming method, luminance of light-emitting elements varies due to variations in the electrical properties of transistors.